PHYSIOLOGY OF CARDIAC MUSCLE 



203 



FIG. b. Mitochondria Ircm lat ventricle showing cristae. 

 X55,200. [From Siekevitz ('2 16).] 



plasmic reticulum which surrounds the sarcomeres 

 at the level of the Z bands appears to communicate 

 with the nucleus also. The mitochondria appear to 

 lie in the sarcoplasm without attachment to the 

 myofibril although, in contrast to other cells where 

 the mitochondria can move freely in the cytoplasm, 

 they are confined within the length of one sarcomere 

 along the myofibrillar axis (igo). Glycogen granules 

 and, more rarely, fat droplets are also seen in normal 

 cardiac muscle cells. 



Myoglobin of Heart Muscle 



Cardiac and red skeletal muscle are characterized 

 by the presence of the red pigment myoglobin in the 

 sarcoplasm. It is a conjugated protein containing an 

 iron porphyrin prosthetic group which is closely re- 

 lated to hemoglobin. Its molecular weight is one- 

 fourth that of hemoglobin (16,500 vs. 66,000) and 

 it resembles but is not identical in amino acid com- 

 position with the monomer of hemoglobin, the latter 

 being composed of four such iron-porphyrin protein 



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 80 



70 



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P 60 



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o 30 

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 10 



10 20 30 40 50 60 70 80 

 OXYGEN TENSION mmHg 



Cytochrome Coronory 

 Oxidase Venous 



90 100 



Arterlol 



FIG. 7. Myoglobin (horse) and hemoglobin (human) 

 oxygen dissociation curves at physiologiccd temperature and 

 pH [From Biorck (24).] 



units. The porphyrins are identical (protoporphx rin 

 IX) (159)- The conformation of myoglobin and the 

 specific folding of its peptide chain has been inten- 

 sively studied by Kendrew (119) using X-ray diflfrac- 

 tion methods. 



The oxygen dissociation cur\e for myoglobin is 

 hyperbolic in comparison with the curve for hemo- 

 globin which is sigmoid (fig. 7). The sigmoid nature 

 of hemoglobin's dissociation curve is due to the inter- 

 action between the four hemoprotein units in the 

 hemoglobin molecule (42). 



From figure 7 it may be seen that at coronary 

 sinus Poii, hemoglobin is only 35 per cent saturated, 

 whereas myoglobin is 85 per cent saturated. At the 

 minimum effective Po-i of cytochrome oxidase, the 

 terminal electron transport enzyme (Pc, = 5 mm 

 Hg) myoglobin is still 60 per cent saturated. This 

 signifies a limited oxygen storage function of myo- 

 globin in heart and other red muscles, although the 

 energy requirement of the mammalian heart is so 

 high that the amount of oxygen stored by cardiac 

 myoglobin would suffice to sustain only six additional 

 cardiac beats at rest in the absence of oxygen and 

 glycolytic energy sources. Of more significance than 

 this oxvgen storage capacity is the ability of myoglobin 

 to facilitate the transport of oxygen through mem- 

 branes (212a). This property would facilitate the 



